The present invention relates to the field of medical imaging applicable to computer-assisted surgery.
A conventional equipment used for computer-assisted surgery, or more specifically computer-assisted surgical acts, includes a three-dimensional localization system that determines the positions an spatial orientations of marking means mounted on instruments, pointers, sensors, or anatomic structures, which localization system may be optical, magnetic, mechanical, ultrasonic, inertial, etc.
In this type of surgical intervention, three-dimensional images of the patient to be operated on are performed prior to the intervention. These images can be performed several days before the intervention and they are performed in the room where the medical imaging equipment (scanner, magnetic resonance MRI, etc.) is. During the surgical intervention, the pre-recorded images and the position of an intervention tool acting on the patient are desired to be simultaneously observed. The pre-recorded images thus have to be transposed to the operating reference frame. In other words, the reference frame of the pre-recorded images has to be readjusted on a reference frame linked to the patient.
For this purpose, conventionally, specific points of the patient or material structures are made to correspond with the same points or structures repeated in the pre-recorded images. Further, once this correspondence has been established once, it is desired to be able to recover it if the patient moves, if he is displaced or upon another intervention. For this purpose, a reference body, the motions of which, followed by a localization system, are determined and used to readjust the pre-recorded images on the patient, is attached to the patient.
Various reference body fastening means are possible.
In neurosurgery and in brain radiotherapy, known systems use a ring placed around the skull and on which are installed tags visible in the localization system. These generally are relatively light so-called stereotactic frames, which are attached by tightening around the head or by fastening in the cranium.
In ENT surgery, known systems use a flexible helmet that encompasses the patient""s head and that is globally tightened around the head, as well as systems using a nasal bearing and pins that are placed in the ears. These two types of systems do not provide a good stability along time and can generate significant inaccuracies, especially if they are disassembled and assembled back on the patient.
In orthopedic surgery, it is known to place in the operated bone rods with screws on which the marking means are then fastened; more specifically, in spine surgery, pliers with jaws that are tightened on the spinal crests of the operated vertebras are generally placed. The marking means are then installed on these pliers. Such fastenings are rigid and invasive.
None of these fastening devices brings the reference body to come to a predetermined position with the patient""s anatomic structure and, to readjust the reference of the pre-established three-dimensional images on the reference frame of the marking means, the position of the reference body with respect to the patient first has to be found, and the patient""s anatomic structures then have to be readjusted on the reference frame of the reference body. This has the disadvantage of requiring localization of anatomic points of the patient and can be a source of errors. Further, if for any reason, the fastening device has to be removed from the patient, it will be necessary to resume this localization of anatomic points of the patient to determine the new relation between the position of the new location of the fastening device and the patient""s anatomic structure.
Besides, fastening devices are known that follow the shape of anatomic structures.
Thus, in general radiotherapy, anatomic mattresses that follow the shape of the patient""s body have been used; such mattresses, for example thermo-inflatable mattresses, in addition to the discomfort that they cause, in practice only provide a poor accuracy.
The use of made-to-measure moulds or templates for each patient such that they coincide with the external shape of an anatomic structure defined on medical images of this structure is also known. This solution has the disadvantage of requiring formation of an individualized template for each patient, which is heavy to implement and increases costs.
Dental castings to be fastened on the patient""s jaw or mandible have also been used; marking means are attached on these dental castings and enable the localization system to find the position of the castings. Here again, this solution has the disadvantage of requiring an individual casting for each patient. Further, a disadvantage of this system is that the dental casting must be pressed against the teeth all along the system use and, besides, the bulk of such castings can be disturbing for certain types of surgery (oral surgery, for example) or for the placing of breathing pipes.
To perform the readjustment, prior art uses three methods.
Thus, in known methods, the reference body is placed on the patient at the time of the imaging examination. This reference body may be one of the devices described hereabove, like the nasal bearing including pins placed in the ears, or material structures, such as screws, balls, rings, stuck against the skin or inserted in a patient""s bone. This reference body is easily found on the obtained images, for example by application programs. The position of the reference body must not vary and these methods have the disadvantage of having to leave, on the patient, the reference body between the imaging examination and the surgical intervention. In practice, it is often necessary to perform a new imaging examination just before the intervention.
In other known methods, the reference body is not placed on the patient during the imaging examination. These methods use a readjustment by means of a number of specific anatomic points of the patient. Upon intervention, an operator or a surgeon points at specific points of the patient with a sensor to find them with respect to the localization system. Application programs find these specific points on the images, which enables readjustment of the pre-recorded image during the intervention. The major disadvantage of such methods is that the finding of anatomic points necessarily is somewhat inaccurate. This can cause significant errors, especially in rotation.
Other more accurate methods use application programs having clouds of points belonging to two surfaces match together. A cloud of points (20 points, for example) is located by the surgeon on an anatomic surface of the patient and the readjustment between this surface and the homologous surface located on the images is performed by application programs using iterative convergence algorithms. Such methods have the disadvantage of requiring long computation times or of being rather unreliable if they do not start from an initial position close to the solution.
An object of the present invention is to provide a method for readjusting the reference frame of a three-dimensional image of a patient on the reference frame of a patient which is easy to implement.
An object of the present invention is to provide a method for readjusting the reference frame of a three-dimensional image of a patient on the reference frame of a patient, which is reliable and allows displacements of the patient.
An object of the present invention is to provide a method that does not require the step of finding, in the operating reference frame, the position of several anatomic points of a patient.
An object of the present invention is to provide a reference body fastening device which can be located by a localization system and which is easily installed.
An object of the present invention is to provide a device that can apply to a large series of patients.
An object of the present invention is to provide a device which has a good stability all along the intervention.
An object of the present invention is to provide a device which has a substantially determined position with respect to the anatomic structure on which it rests.
To achieve these objects, the present invention provides a method for readjusting the reference frame of a device placed on a patient during an intervention with respect to the reference frame of a pre-recorded three-dimensional image of this patient. The method includes the steps of:
a) placing on the patient a device applicable to several patients and having a predetermined position relation with the patient""s anatomic structure;
b) finding the position of said device with respect to a localization system; and
c) by means of the localization of said device and of the predetermined position relation of the device with the patient""s anatomic structure, determining a relation between the reference frame of the device and the reference frame of said image.
According to an embodiment, the step of determining the relation between the reference frame of the device and the reference frame of said image includes the steps of:
determining, with respect to said device, the position of points belonging to the patient""s anatomic surface,
using iterative readjustment algorithms to determine in the pre-recorded image the position of this anatomic surface, these algorithms using the predetermined position relation of the device with the anatomic structure to begin their search, and
determining a transfer array defining the relation between the reference frame of the device and the reference frame of said image.
According to an embodiment, in the case where the patient keeps a substantially stable position during the imaging, the readjustment algorithms use, whoever the patient, a single initial transfer array determined once and for all by means of said device.
According to an embodiment, the readjustment algorithms use a set of initial transfer arrays limited to the space of translations.
According to an embodiment, the readjustment algorithms use an initial transfer array and, in the case where said device enables determining the three rotation parameters of the initial transfer array, the three rotation parameters of the array are defined by choosing an arbitrary point on the patient""s surface defined on the three-dimensional image, displaying this point according to cross-section and projection views, and asking an operator to designate this point in the reference frame of the device by means of a sensor.
According to an embodiment, the readjustment algorithms use an initial transfer array and, in the case where said device enables determining certain parameters of the initial transfer array, the other parameters are defined by performing a fine readjustment based on the admissible positions so calculated and only keeping the best relation.
The present invention also provides a fastening device that can apply to a wide series of patients and support a marking means; this device includes at least three bearings intended for resting upon an anatomic structure of the patient so that a predetermined position relation between the device and the anatomic structure can be established.
According to an embodiment, the device includes:
an element including a first V-shaped bearing intended for resting upon the nasion and two other bearings intended for resting upon the forehead above the eyebrows;
first and second moulds following the shape of the patient""s ear conchas, intended for being respectively placed in one or the other of the patient""s ears;
a holding bar for holding said element in a stable position, said holding bar including a central portion intended for resting upon the element and two branches intended for being coupled to said first and second moulds.
According to an embodiment, the device further includes sliding rings having rods for coupling said branches to said first and second moulds.
According to an embodiment, said element has a multiple-position notch system located between the first bearing and the two other bearings, the notch system being intended for receiving the central portion of the holding bar and for enabling setting of the device hold pressure.
According to an embodiment, the marking means includes two lateral plates oblique with respect to each other exhibiting reference elements.
According to an embodiment, the marking means further includes a front triangular plate having at least three holes and/or at least one calibration pin.
According to an embodiment, the device, applicable to a lumbar vertebra, includes two bearings intended for being placed on the back edge of the spinal crest and pliers with a jaw perpendicular to the straight line passing through said two bearings.
According to an embodiment, the device, applicable to a thoracic vertebra, includes two bearings intended for being placed on the back edge of the spinal crest and pliers with a jaw forming, with respect to the straight line passing through the two bearings, a predetermined angle defined by the level of the vertebra.
The present invention also provides a reference pin, usable in the method of the present invention or in cooperation with the device of the present invention; the reference pin includes:
a cylindrical tip having a given diameter D, intended for inserting into a hole drilled in a bone and substantially having the same diameter D,
a flange supported by the cylindrical tip, and
a marking means enabling localization of a reference point of the reference pin.